The present invention relates in general to the manufacture of imaging films and elements and in particular to a novel method of controlling necking-in of a polyester film during machine direction stretching in a biaxial orientation process.
The conventional processes to manufacture polyester photographic film supports typically involve biaxially orienting a cast amorphous sheet. Such orientation processes and subsequent heat setting significantly improves the flexibility, tensile strength, dimensional stability, etc. of materials such as Polyethylene Terephthalate (PET) and Polyethylene Naphthalate (PEN) and thus renders them usable as photographic film supports.
It is well known to those skilled in this field that it is necessary to stretch the cast sheet in both the longitudinal or machine direction (MD) and the transverse or width direction (TD) beyond its strain hardening point to produce films with uniform thickness and useful properties. A polyester machine is thus often designed to provide enough stretching latitude to accomplish this.
However, it would be difficult, if not impossible, to use an existing machine that was designed to manufacture PET supports, for example, to manufacture film supports using materials such as PEN without significant modification of the machine. This is primarily due to the fact that PEN has a much higher strain hardening point than that of PET and therefore requires higher stretching ratios. The existing width of the casting wheel, extrusion die, machine direction orientation equipment and the tenter frame dictate the stretch ratios achievable on each machine. If the maximum ratios achievable on the existing film machine are lower than the ratios required to pass the strain hardening points, one would have to rebuild the machine. For example, even increasing just the final tenter width would be a problem as films would have to have excessive edge trims to make film rolls with the same width as was made before the increase. One alternative would be to change the casting wheel, die, machine direction orientation equipment, but this would be expensive and in any case, there would be difficulty maintaining flexibility on the film line to manufacture PET as well as PEN films.
Another alternative would be to slit some portion of the MD stretched sheet before entering the TD stretch. However, this would incur excess edge-trim waste and cause robustness issues caused by slitting a film while it is still mostly amorphous and much thicker than the final film thickness for photographic film supports. These are both undesirable alternatives.
It would be desirable to have a process or a machine that is able to control the width of different types of polyester (e.g., PET and PEN) film supports during the machine direction orientation process step.
The present invention discloses a process to manufacture biaxially oriented imaging polyester film supports by allowing the edges to xe2x80x9cneck inxe2x80x9d during the longitudinal or machine direction stretch in a controlled manner to minimize edging waste and to minimize edge effects. As used herein, the term xe2x80x9cneck-inxe2x80x9d means reduction in width of a cast sheet when conducting machine direction (MD) orientation.
To solve the problem outlined above, the present invention takes advantage of the natural neck-in tendency of a thermoplastic material upon stretching in the machine direction. That is, with use of the present invention, the width of a MD stretched sheet is reduced in a controlled manner by a desired amount such that no edge trim is necessary. The present invention thus addresses the needs related to a MD orientation process where the film width leaving the process can be controlled to eliminate edge trimming after MD orientation, reduce or eliminate edge orientation effects, avoid large capital expenditures to the film line while maintaining the ability to manufacture different types of polyester film supports, such as PET and PEN, alternatively on the same film line.
Hence, the present invention discloses:
a method for controlling the width of a polyester film support comprising, in order, the steps of:
(a) extruding a polyester resin using one or more extruders;
(b) casting the resin on a casting wheel to form a polyester sheet;
(c) stretching the polyester sheet along the machine direction by ratios of 3 to 5 times at temperatures ranging from above the glass transition temperature to below the thermal crystallization temperature, wherein the machine contains extended edge rollers (14) in the drafter at the point where stretching occurs;
(d) stretching the polyester sheet along the transverse direction by ratios of 3 to 5 times at temperatures ranging from above the glass transition temperature to below the thermal crystallization temperature; and
(e) heat setting the polyester sheet at temperatures ranging from 180 to 250xc2x0 C.
Also disclosed is a beaded drafter modified by having extended rollers (14) installed to narrow the width of a polyester sheet during the machine stretch as it passes along the drafter.